(1) Field of the Invention
The present invention relates to an on-board device for measuring the weight and even the position of the center of gravity of an aircraft, e.g. a rotorcraft.
(2) Description of Related Art
These two items of information are critical factors for safe and effective operation of any aircraft. Before each flight, the pilot of a vehicle of that type must therefore make sure that the weight and the balance of the aircraft relative to its center of gravity do not exceed limits that are predefined by the manufacturer.
A first device is known that is not mounted on board an aircraft and that seeks to weigh the aircraft when empty, i.e. without any load (passengers, fuel, . . . ), by arranging the aircraft under very precise conditions on a plurality of jacks. Those jacks are arranged on specific supports to avoid introducing interfering forces. The empty weight is then added to the load weight so as to estimate the total weight of the aircraft prior to takeoff and so as to estimate the position of its center of gravity.
Since measuring the empty weight is very difficult to perform, it cannot be done prior to each flight, given the means and the time required.
Devices on board an aircraft are sometimes used to obtain an accurate measurement of the weight and the position of the center of gravity before each flight. In addition, such devices improve flight safety, in particular since the pilot can verify unreliable information coming from external sources that relates to the loading weight and to the position of the center of gravity.
Those on-board devices rely on determining the deformations of a portion of each undercarriage and on easily deducing the total weight and the position of the center of gravity of the aircraft therefrom.
Document U.S. Pat. No. 3,426,586 describes a device provided with a deformable tube having strain gauges, with one such tube being inserted inside the axle of each undercarriage. The deformation of each tube as measured by the strain gauges makes it possible to deduce the total weight of the aircraft and the position of its center of gravity.
That device is advantageous. Nevertheless the field of aviation gives rise to requirements that are strict, in particular in terms of reliability. Unfortunately, strain gauges are sensitive to temperature, and thus they require the use of specific means for refining the measurements and for recalibrating them.
Document U.S. Pat. No. 4,312,042 suggests measuring the bending angle of an element of each undercarriage to obtain a direct indication concerning the weight of an aircraft. That document teaches using two inclinometers in order to determine the bending of the element in question.
Nevertheless, experience shows that it can be difficult to arrange two inclinometers on each undercarriage.
Document U.S. Pat. No. 3,488,997 describes a device for measuring the weight and the position of the center of gravity of an aircraft having an undercarriage that possesses at least one ground-contacting member that has a deformable element.
In addition, a bar is arranged inside the deformable element. Two sensors arranged on the deformable element via arms measure the movement of the bar respectively at two ends of the bar via arms.
When a force is exerted on each ground-contacting member, a free first portion of the deformable element carrying a first sensor deforms, while a second portion of the deformable element carrying a second sensor remains relatively stationary. Those two sensors are thus arranged on the deformable element to measure the movement of the deformable element via arms.
Document U.S. Pat. No. 3,488,997 neither discloses nor suggests two sensors arranged at the same end, nor does it suggest processor means for generating an alert.
Document FR 2 564 582 implements a rod arranged in an axle and two inductive sensors arranged on either side of the rod in order to perform differential detection of variations in relative position between the rod and the axle.
Document EP 0 482 289 implements an inductive sensor having two coils and a strip of ferromagnetic material that is movable in the coils.
Document FR 2 875 598 describes an on-board device for measuring the weight and the position of the center of gravity of an aircraft that has a plurality of undercarriages. Each undercarriage is provided with at least one ground-contacting member having an element that is deformable under the action of the weight of the aircraft when the aircraft is standing on a surface. The deformable element is provided with a bar having an eddy current sensor at its free end, the bar being arranged inside the deformable element.
Document WO 2004/074787 describes a device having a first portion and a second portion with an elongate member being attached to the first and second portions.
In addition, the elongate member has a magnetized region. At least one magnetic field probe element is used for detecting a distortion of the magnetic field emanating from the elongate member when a load is exerted on one or the other of the portions.
In one embodiment, at least one pair of probes is used, the signal representing the bending moment of the elongate member corresponding to the difference between the signals from the two probes of a pair.
Document U.S. Pat. No. 5,172,781 is remote from the technical field of undercarriages. That document presents a bed having a weight-measuring device. The device has a plurality of sensors known under the acronym “LVDT” for “linear variable differential transformer”. Such a sensor is referred to below as an “LVDT sensor” for convenience.
The signals delivered by the various LVDT sensors are then processed in order to generate a single signal representing the average of the movements measured by the LVDT sensors.
Conventionally, an LVDT sensor comprises a ferromagnetic core and a set of coils surrounding the core. The set of coils has a primary coil and two secondary coils, with the core being suitable for co-operating with a feeler.
Under such circumstances, the movement of the feeler causes the core to move and consequently modifies the coupling coefficients between the primary coil and the secondary coils.
Such an LVDT sensor presents the advantage of being relatively insensitive to electromagnetic disturbances.
In addition, it is possible to detect an electrical failure of an LVDT sensor. Nevertheless, a mechanical failure, such as a break between a feeler and an LVDT sensor core is difficult to detect.
Document U.S. Pat. No. 4,503,922 describes a device having a deformable strip arranged between two plates, each plate having a winding.